Replace gethrtime() with cpu_ticks(), as source of random for the taskqueue
selection.  gethrtime() in our port updated with HZ rate, so unusable for
this specific purpose, completely draining benefit of multiple taskqueues.

MFC after:	2 weeks

(cherry picked from commit 514d9dd780e4da539aa8f76983d7801714ca0b8c)
- Naively fix build by partially reverting r267029 to still use
  gethrtime() when building libzpool.

X-MFC-With:	267029
(cherry picked from commit dea49bcbae39f8c448d248609b6ef08fa60f418b)